This invention relates to a backplane for performing a number of functions, including processing of radio frequency signals.
Backplanes are well-known devices in the electronics field that are typically used to provide interconnection and interfacing between various components and circuits. They often employ one ore more layers utilizing printed circuits, such as copper traces, to provide the interconnection. In multi-layer backplanes, the traces extend between layers to make connections between different circuits. One use for a backplane is with circuits that convey radio frequency signals. Such backplanes often employ one or more coaxial connectors. One type of backplane using coaxial connectors is shown in U.S. Pat. No. 5,641,294 granted Jun. 24, 1997. The backplane of that patent is to accommodate one or more circuit cards that are to be plugged into it and includes a plurality of coaxial connectors. The connectors are precisely aligned by mounting each of them on the backplane in a resilient bushing which permits limited connector movement.
In any piece of electronic equipment in which radio frequency signals are present, a problem arises where a plurality of coaxial connectors are located adjacent one another, in that leakage of the radio frequency energy occurs between the connectors. This results in possible interference between the circuits associated with the connectors. Also, leakage of the energy between one or more of the connectors and ground possibly results in making the RF signal noisy or otherwise corrupting it. Consideration also must be given to matching the impedance of certain components mounted on the backplane to that of other components, such as cables, which may be external to the equipment. In a backplane using printed circuit technology and having traces of conductive material, leakage of the radio frequency energy can also occur between the traces and cause signal interference. The foregoing ""294 patent gives no consideration to these problems.
The present invention is directed to a backplane of the multi-layer type for handling radio frequency signals which is constructed to overcome the aforesaid problems. The backplane also can accommodate other signals, such as digital data, and provide interconnection to power supply sources.
In accordance with the invention, the backplane has a plurality of layers with printed circuit traces, each for performing a particular function. For example, a printed circuit trace layer can be configured as or associated with a splitter of the radio frequency energy between various transmitter sources and/or one or more receivers, a coupler to couple signals from two or more sources, and so forth. Each of the printed circuit trace layers, has an associated ground plane layer of electrically conductive material separated from the trace layer by an electrical insulator, such as a sheet of dielectric material. A sheet of the dielectric material separates the conductive ground plane layer from the next layer of circuit traces.
The uppermost backplane layer, hereafter called the interface layer, is a combination of ground plane and circuit traces and has sockets for a plurality of coaxial connectors and for connection of other circuit components. The interface layer is separated from the next conductive trace layer by an electrical insulator, e.g., a dielectric material sheet. The backplane thereafter alternates with a trace layer, dielectric sheet, ground plane layer, dielectric sheet, trace layer for a desired number of iterations to the last ground plane layer for a trace layer. The backplane is completed by adding a dielectric sheet to the ground plane layer of the lowermost circuit trace layer and finally a conductive xe2x80x9ccapxe2x80x9d layer which caps, or completes, the backplane with an overall ground plane.
The center pin of each coaxial connector extends into the backplane to make electrical contact with a trace on a selected one of the printed circuit trace layers. The ground part of each connector is electrically connected to each ground plane layer through which it passes and the ground plane layer surrounds the center pins of the coaxial connectors passing therethrough. The ground plane cap layer is electrically connected to one or more of the other ground plane layers and prevents unwanted radiation of RF energy between adjacent connectors.
In a preferred embodiment of the invention, the backplane is also provided with vias, which are through-holes plated with an electrically conductive material. The vias extend from the upper interface layer to the lowermost ground plane layer of a trace layer. The vias preferably are spaced apart, similar to a stitching pattern, and are located between traces of the trace layers to also prevent leakage of the radio frequency energy from one trace to another. The spacing of the vias is selected as a function of the wavelength of the energy carried by the associated trace. Also, the invention controls the impedance of the connectors by removing conductive material from layers below the terminal end of a connector center pin and from transition vias below a point of connection to a trace, down through the cap layer.